This application claims benefit of priority under 35USC xc2xa7119 to Japanese Patent Application No. 2000-69820, filed on Mar. 14, 2000, the entire contents of which are incorporated by reference herein.
The present invention relates to a two-wavelength laser device which includes a front end face film and a high-reflectivity multilayered film and performs two-wavelength oscillation.
Optical disk systems currently put into practical use are roughly classified into a system which records and reproduces data into and from a compact disc and a DVD system which performs data recording and reproduction at higher density. An optical semiconductor laser used for a compact disc recording medium has an oscillation wavelength of 780 nm. An optical semiconductor laser used in the DVD system has an oscillation wavelength of 650 nm. To obtain a high optical output, each of these optical semiconductor lasers has a front end face anti-reflectivity film and a rear end face high-reflectivity film on its end faces, thereby efficiently extracting light, emitted from the rear surface of a resonator, from the front surface. The thicknesses of these front end face anti-reflectivity film and rear end face high-reflectivity film are calculated on the basis of the oscillation wavelength of each laser.
Recently, disk apparatuses including a high-density recording medium such as a DVD in addition to a CD-R, CD-RW, and the like have appeared. Some disk apparatuses of this type incorporate both an optical semiconductor laser having an oscillation wavelength of 780 nm and an optical semiconductor laser having an oscillation wavelength of 650 nm. However, since optical systems are required to shrink as disk apparatuses are miniaturized, two-wavelength lasers including two resonators having the above-mentioned two oscillation wavelengths in a single crystal structure are most often used.
In this two-wavelength laser, however, the film thicknesses of the front end face anti-reflectivity film and rear end face high-reflectivity film must be respectively matched with their wavelengths xcex. This introduces inconveniences to the fabrication steps. FIGS. 32 and 33 show end face film formation steps relevant to the present invention. As shown in FIG. 32, semiconductor laser diodes having an oscillation frequency of 650 nm and semiconductor laser diodes having an oscillation frequency of 780 nm are alternately formed on a single chip. The front end faces of the 650-nm laser diodes 51 are exposed, and their other portions and the 780-nm laser diodes are covered with a mask 52. A single-layer reflecting film 54 is formed by sputtering on a laser emission portion 53 of each exposed end face. The film thickness of this single-layer reflecting film 54 is calculated on the basis of an oscillation frequency of 650 nm. Subsequently, as shown in FIG. 33, the mask 52 is moved to expose laser emission regions 56 at the front end faces of the 780-nm laser diodes 55, in order to form a thin film having a predetermined film thickness on these front end face emission regions 56. After that, a single-layer reflecting film having a film thickness calculated on the basis of an oscillation frequency of 780 nm is formed on the exposed portions.
In the above fabrication steps, the spacing between the two semiconductor lasers is set to around 100 xcexcm in accordance with the effective dimensions of these optical semiconductor elements and the requirements of an optical system into which these optical semiconductor elements are incorporated. Therefore, the fabrication method which forms end face films by using the mask 52 is inefficient because the method requires highly accurate microfabrication. The working efficiency is also low because thin film formation is performed for each semiconductor laser. The working efficiency is similarly low when an end face high-reflectivity film is formed on the rear end face of each semiconductor laser. Furthermore, the mask 52 is very difficult to move since the planarity of the element surface is disturbed by the multilayered thin films already formed.
As a method of forming thin films without using any shielding masks, a thin film formation method using optical CVD or the like described in patent gazette (U.S. Pat. No. 2,862,037) is used. However, this method has the following problems. In the method using optical CVD, as shown in FIG. 34, a light amount control ND filter 62 for controlling the thin film growth rate is placed between a light source and the end face of a laser diode 61 on which a thin film is to be formed. Light emission regions 53 and 56 having the different oscillation wavelengths as described above are arranged with fine intervals between them. Therefore, lights 63 passing through the ND filter 62 must exactly irradiate desired light emission regions 53 and 56. Accordingly, the ND filter 62 and the light emission regions 53 and 56 require an extremely high level of positional adjustment. Any adjustment difference produces an error in thin film formation by a change in the light amount, and this greatly lowers the productivity.
Also, in the two formation steps described above, the structures of jigs and the mechanism of a reaction tank inside the film fabrication apparatus are elaborated in the process of forming films on optical semiconductor lasers. This degrades the flexibility of the apparatus.
It is, therefore, an object of the present invention to provide a two-wavelength semiconductor laser device having high reliability, meeting the necessary performance, and capable of forming a high-productivity end face reflecting film at one time.
A semiconductor laser device of the present invention is characterized by comprising a substrate, a first laser element portion formed on the substrate to oscillate laser light having a first wavelength, a second laser element portion formed on the substrate to oscillate laser light having a second wavelength, a front end face film formed at once on front end faces of the first and second laser element portions and having a uniform film thickness, and a rear end face film formed at once on rear end faces of the first and second laser element portions, having a uniform film thickness, and comprising a plurality of thin films, wherein the film thickness of the front end face film and the plurality of thin films of the rear end face film have an optical length d=(xc2xc+j)xc3x97xcex(j=0, 1, 2, . . . ) with respect to a mean wavelength xcex of the first and second wavelengths. This device is characterized in that the front end face film has a reflectivity of 3 to 37%, and the rear end face film has a reflectivity of not less than 75%. The device is also characterized in that the front end face film is made of a low-refractive-index material having a refractive index n less than 1.8, and the rear end face film comprises stacked layers of thin films made of a low-refractive-index material having a refractive index n less than 1.8 and thin films made of a high-refractive-index material having a refractive index n greater than 1.9. Furthermore, the front end face film is made of Al2O3, and the rear end face film comprises stacked layers of thin films made of Al2O3 or SiO2 as a low-refractive-index material and thin films made of SiN4 or Si as a high-refractive-index material.
A semiconductor laser device fabrication method of the present invention comprises the step of forming, on a substrate, a first laser element portion which oscillates laser light having a first wavelength, forming, on the substrate, a second laser element portion which oscillates laser light having a second wavelength forming a front end face film having a uniform film thickness at once on front end faces of the first and second laser element portions by using electronic cyclotron resonance (ECR) sputtering, and forming a rear end face film having a uniform film thickness and comprising a plurality of thin films at once on rear end faces of the first and second laser element portions by using ECR sputtering.
The present invention can provide a semiconductor laser device having high reliability, meeting the necessary performance, and also having high productivity. It is also possible to provide a semiconductor laser device fabrication method capable of forming an end face film at once and thereby capable of reducing the number of fabrication steps and saving the space of the film formation apparatus.